Disappointing speed.. new line for shorter line length?

I live in a residential street about 1 km from the telephone exchange, there are numerous cabinets up and down the street and yet the best I can get on BT Infinity is just under 30 mbit/sec... I appreciate this is still considered "good" but I wonder why I can't get the full 40 (and/or 76) considering how close the cabinets appear to be... I wondered if I got BT to install a new line there's a possibility that could end up with a shorter line length and I'd get better speeds?

Is there any definitive way to find out which cabinet I'm using and exactly how far it is? I've called BT but all I get is the "you're within our acceptable speed, we can send you a leaflet, blah blah..."

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